docs: def get_subgraphs_by_citation(graph):
      """Stratify the graph based on citations.
  
      :type graph: pybel.BELGraph
      :rtype: dict[tuple[str,str],pybel.BELGraph]
      """
      rv = defaultdict(graph.fresh_copy)
  
      for u, v, key, data in graph.edges(keys=True, data=True):
          if CITATION not in data:
              continue
          dk = data[CITATION][CITATION_TYPE], data[CITATION][CITATION_REFERENCE]
  
          rv[dk].add_edge(u, v, key=key, **data)
  
      cleanup(graph, rv)
  
      return dict(rv)